When health problems led Catholic Bishop of Dunedin Len Boyle to step down in 2002, the comparison was made that Pope John Paul II was in a greater state of physical frailty.

The bishop said he would have empathised had John Paul resigned, but did not hide that he would have been dismayed.

John Paul did labour on to his dying day, in the tradition of the role. Surely no-one saw the best and worst of that approach than Joseph Ratzinger, his right-hand man, regarded as a hardliner even by Catholic standards who in turn became Pope Benedict XVI.

With his own health failing, Benedict has shocked his church by stepping down with the most simple explanation - that the job required "both strength of mind and body" in a combination he could no longer provide. Not since the Middle Ages has a pope made this call. Some should have.

Like his predecessor, Benedict has been a doctrinal and social conservative on issues like abortion, euthanasia, homosexuality and contraception.

He was said to be more active than he was given credit for in confronting the scandal of paedophile priests, but his reluctance to haul offenders to the police rightly angered victims and the wider public. Between these hard lines, his papacy will be remembered as having some positive value.

His replacement is an issue that goes beyond personalities or geographical politics. Choosing a pope is not like choosing whose turn it is to host the Olympics.

There are many within the church who want a more progressive pope. And there are many who don't.

Benedict was elected after John Paul II created many new cardinals from the more conservative ranks of the church and he lowered the threshold from two-thirds support to a simple majority. Benedict restored the two-thirds rule, which means candidates will be more easily blocked, increasing the chances of a compromise.

But Vatican reporter Marco Tosatti points out it is not all politics and strategies when the cardinals meet to vote. These are old men aware they will face judgment soon enough themselves. They really try to imagine the challenges facing the church and pick the man who can meet them, he says.

That would be good. But you'd have to say a measure of human fallibility has sometimes crept into what is meant to be a matter of consulting the will of God.